Handle constant completion when typing path separator

[vinistock]

Motivation

Start showing constant completion when the user types ::.

The reason we didn't support it before is quite interesting. This code: Foo:: is not a constant path. Ruby allows singleton methods to be invoked both with a dot and with the double colon, so that node is actually a method call with no message associated to it yet.

This is the reason why we didn't support the completion yet - because we don't have special handling for showing constant completion when we discover that partial method call node.

Implementation

We haven't implemented singleton completion yet, so for now the implementation for this is actually simplified in a certain way. The correct behaviour is to display both constants and singleton methods when the user types ::, because we can't know ahead of time what the user is trying to type.

For now, we just check if the operator is a :: with a constant receiver and then we display matching constants for completion.

Automated Tests

Added tests.

Manual Tests

1. Start the LSP on this branch
2. Open a file and type RubyLsp:. Then stop typing and dismiss the completion dialogue that was opened when you first typed the R character (click ESC)
3. Now type the second colon and verify that the completion list is brought back

[andyw8]

Open a file and type RubyLsp:. Then stop typing and dismiss the completion dialogue that was opened when you first typed the R character (click ESC)

The completion dialog disappears after I type the first :, but that's what I'd expect.